1. Field of the Invention
The present invention relates to a fuel assembly having fuel rods containing gadolinium as a burnable poison, and nuclear reactor. In particular, the present invention relates to a fuel assembly and nuclear reactor in which, without damaging thermal margin of the fuel rods, fuel economy is improved.
2. Description of Related Art
In a light-water type power reactor in which uranium or uranium dioxide is used as a nuclear fuel material, a fuel assembly is set at a necessary initial uranium enrichment, with the progress of burnup an excess reactivity of the reactor being decreased. From a viewpoint of safety, for the excess reactivity not to be too large, a material called a burnable poison of which negative reactivity decreases with the burnup is added to the fuel. Mainly in a boiling water reactor (BWR) and sometimes in a pressurized water reactor (PWR), gadolinium of atomic number 64 is used as the burnable poison, the gadolinium being added in the form of gadolinia that is an oxide of gadolinium to the nuclear fuel material.
A sectional structure of an existing fuel assembly is shown in FIG. 19.
In the fuel assembly 21, fuel rods 22 and 23 in which the nuclear fuel material such as uranium or the like is enclosed are arranged in a square grid pattern, for instance. A water rod 24 through the inside of which non-boiling cooling water flows during output operation is disposed in the neighborhood of the center. Furthermore, an entirety thereof is accommodated in a channel box 25. In the figure, reference numeral 22 denotes a fuel rod that does not contain gadolinia, reference numeral 23 denoting a fuel rod (hereinafter referred to as gadolinia fuel rod) that contains gadolinia, respectively.
Usually, for four sets of fuel assemblies 21 each, one set of control rod 26 is disposed to constitute a reactor core. Inside the control rod 26, a plurality of poison rods 27 in which a neutron absorber is enclosed are regularly arranged to be accommodated. A length, a number of pieces, a shape and an arrangement of the fuel rods 22 and 23, and an existence, a number of pieces, a shape, an arrangement or the like of the water rod in the fuel assembly are different according to nuclear reactors. The structure of an existing fuel assembly is not restricted to the fuel assembly shown in FIG. 19.
FIG. 20 is a sectional view in a vertical direction (axial direction) showing a structure of a fuel rod. In the fuel rods 22 and 23, at the uppermost portion of a body in which fuel pellets 28 are regularly piled up, a plenum spring 29 is disposed to suppress a displacement of the pellets, the entirety being accommodated in a shield tube 30. Lower and upper end plugs 31 and 32 seal lower and upper portions of the shield tube 30 respectively, inside of the shield tube 30 helium gas being enclosed under an appropriate pressure.
For the fuel rods, according to the position inside the fuel assembly, enrichment of fissile material and concentration (content) of gadolinia are determined.
FIG. 21A is a transversal sectional view showing an arrangement of the fuel rods when the control rod 26 is disposed in the upper left position. Reference marks 1, 2, 3, 4 and 5 denote the fuel rods 22 that do not contain gadolinia, G1, G2 and G3 denoting gadolinia fuel rods 23, respectively. WR denotes the water rod 24.
FIG. 21B is a diagram showing, in a vertical direction (axial direction) of the fuel rods (1, 2, 3, 4, 5, G1, G2 and G3), enrichment distributions of the fissile material (uranium for instance) and concentration distributions of gadolinia. In the figure, there are used the enrichments of uranium of A to G, and the gadolinia concentrations (additional ratio) of a to c. The gadolinia fuel rods 23 contain uniformly in the axial direction uranium of the enrichment of, for instance, B (the enrichment C is also satisfactory). Only the gadolinia concentrations are shown in FIG. 21B.
The uranium enrichments decrease in the order of A greater than B greater than C greater than D greater than E greater than F greater than G, G being the enrichment of natural uranium. In the fuel rod 22 that does contain no gadolinia, there are disposed at the upper and lower ends the portions (natural uranium blankets) that contain only the natural uranium. There may be cases where the uranium enrichments have a difference in the axial direction.
The gadolinia fuel rods 23, though usually disposed at the positions other than that of the outermost periphery of the fuel assembly, may be disposed at the positions of the outermost periphery. The gadolinia concentrations are in the decreasing order of a greater than b greater than c, a difference being given in some cases in the axial direction. Furthermore, whereas there are cases where the natural uranium blankets are disposed at the upper and lower ends, without the natural uranium blanket there may be disposed gas reservoirs that are called plenum space at the upper and lower ends.
Another existing arrangement of the fuel rods in the fuel assembly 21 is shown in FIG. 22.
FIG. 22A is a transversal sectional view showing an arrangement of the fuel rods when the control rod 26 is located in the upper left position. Reference marks 1, 2, 3, 4, 5, 6 and V1 and V2 denote the fuel rods 22 that contain no gadolinia, G1, G2 and G3 denoting the gadolinia fuel rod 23. The fuel rods designated by reference marks 1, 2, 3, 4, 5, 6, and G1, G2 and G3 are long-length fuel rods, the fuel rods designated by the reference marks V1 and V2 being short-length fuel rods shorter than the long-length fuel rods in fuel effective portion. WR denotes the water rod 24.
FIG. 22B is a diagram showing, in the vertical direction (axial direction) of the fuel rods (1, 2, 3, 4, 5, 6, V1, V2, G1, G2 and G3), enrichment distributions of the fissile material (uranium for instance) and concentration distributions of gadolinia.
In the figure, there are used the uranium enrichments of from A to G and the gadolinia concentrations of from a to c. The uranium enrichments are in the decreasing order of A greater than B greater than C greater than D greater than E greater than F greater than G, G being the enrichment of the natural uranium. In the long-length fuel rods, there may be disposed the portions that contain only the natural uranium at the upper and lower ends. Further, in the axial direction, the uranium enrichments may be differentiated. Though the gadolinia fuel rods 23 are usually disposed in the positions other than the outermost periphery of the fuel assembly, those may be disposed in the positions of the outermost periphery. The gadolinia concentrations are in the decreasing order of a greater than b greater than c, the concentrations being differentiated in the axial direction in some cases.
The fuel assembly shown in FIGS. 22A and 22B is designed to be suitable for the following fuel grid pattern (hereinafter referred as D lattice). That is, in the D lattice, a width of the non-boiling water region between outer walls of the channel boxes of the adjacent fuel assemblies is configured to be larger in a control rod insertion side than in the opposite side (non-insertion side). The fuel assembly is loaded with a different spacing from adjacent fuel assemblies in the reactor core comprising D lattice.
In the D lattice, the control rod insertion side, being larger in thermal neutron flux distribution in comparison with the non-insertion side, is likely to be high in local power. Accordingly, when a transversal cross-section of the fuel assembly is divided into two regions of a control rod side and an opposite-control rod side, in the fuel rods disposed in the region of the control rod side, in comparison with the fuel rods disposed in the region of the opposite-control rod side, the enrichment of the fissile material (uranium) is lowered. For instance, in the fuel rods 22 containing no gadolinia that are disposed at the outermost periphery on the opposite-control rod side, the uranium enrichment is set at the highest value of A. Whereas, those of the fuel rods 22 containing no gadolinia that are disposed in the region on the control rod side are set at B, C and D, all lower than A. Furthermore, also in the gadolinia fuel rods 23, the uranium enrichments of those disposed in the region of the opposite-control rod side are set at the most highest value of A. Whereas, in the gadolinia fuel rods on the control rod side, the uranium enrichments are set at B and C lower than A. The uranium enrichment averaged over the fuel assembly (bundle) is set at 3.96 wt %.
Now, in the existing fuel assemblies shown in FIGS. 21 and 22, respectively, natural gadolinium is used as the burnable poison, the natural gadolinia that is an oxide of the natural gadolinium being added to the nuclear fuel material. That is, the natural gadolinium is a mixture of six kinds of isotopes of which mass numbers are 154, 155, 156, 157, 158 and 160, the content ratios (isotopic composition) of the respective isotopes being 2.1 wt %, 14.5 wt %, 20.3 wt %, 15.7 wt %, 25.0 wt % and 22.5 wt %. Among these, gadolinium of mass number 157 (Gd-157) has the largest thermal neutron absorption cross section in all nuclides, gadolinium of the mass number of 155 (Gd-155) having such a large thermal neutron absorption cross section as approximately one fourth that of Gd-157. Furthermore, the thermal neutron absorption cross sections of Gd-156 and Gd-158 generated due to absorption of the neutron by Gd-155 and Gd-157 are approximately one several tens thousandth those of Gd-155 and Gd-157.
Thus, when the neutron absorption cross section of its own is large and that of a daughter isotope generated due to the absorption of the neutrons is small, due to the neutron absorption (burnup) the absorption cross section decreases largely, the isotopes being applicable as the burnable poison. The total content of Gd-157 and Gd-155 in the natural gadolinium is approximately 30 wt %.
The addition amount of gadolinia is set so that, in addition to appropriately suppressing the excess reactivity, the negative reactivity due to the burnable poison continues up to a cycle end. Furthermore, in addition to the above, the addition amount of gadolinia is set so that the poison reactivity remains at the cycle end to make unnecessary to increase the enrichment of the fissile material. According to the number of operation month per cycle, power density or the like, the concentration of gadolinia needs to be controlled.
In the existing fuel assembly, as shown in the following, there is a problem of residual reactivity due to Gd-155, Gd-157 or the like at equilibrium concentrations. That is, the gadolinia concentration is set so that at the cycle end, the poison reactivity does not remain. However, relatively high contents of Gd-154, Gd-156 or the like are contained in the natural gadolinium, and Gd-155 and Gd-157 generated due to the neutron absorption of Gd-154 and Gd-156 exist in equilibrium concentrations. Accordingly, there remains the poison reactivity due to Gd-155 and Gd-157 at the cycle end,.
Furthermore, in the gadolinia fuel rods, uranium oxide or the like, which is the nuclear fuel material, and gadolinia are mixed to form a solid solution. Accordingly, thermal conductivity thereof is lower than that of uranium oxide alone. In particular, the higher the concentration of gadolinia is, the larger the degree of decrease of thermal conductivity is. When the thermal conductivity decreases, even if a linear power density (thermal power per unit length of the fuel rod) is equal, temperature of the fuel rod goes up to be likely to adversely affect on the performance of the shield tube. In order to avoid these problems, in the existing fuel assembly, the enrichment of the fissile material such as uranium in the gadolinia fuel rod is made lower than that of the fuel rod that contain no gadolinia to lower the power. Thereby, care is taken so that the temperature of the fuel rod does not go up excessively.
On the other hand, for the uranium enrichment, from the restriction on critical safety during manufacture and transport, the maximum value is determined. Since the uranium enrichment of the gadolinia fuel rod is restricted, there occurs a problem that the uranium enrichment averaged over the entire fuel assembly cannot be sufficiently increased. Such problems accompanying the use of gadolinia become particularly conspicuous in an initial loading core of which gadolinia concentration is large and a core of a longer operation time period.
Furthermore, when disposing the gadolinia fuel rods in the following positions, the uranium enrichment is necessary to be low.
(1). A region on a control rod insertion side in the D lattice.
(2). Positions of four corners of the fuel bundle disposed in a second position inwardly from the outermost periphery.
(3). Positions adjacent to the short-length fuel rods.
(4). Positions of the outermost periphery, in particular of four corners thereof.
That is, in the D lattice, the region on the control rod insertion side is larger in the thermal neutron flux distribution than the region on the non-insertion side is, the local power tending to be higher. Furthermore, in the aforementioned general design, the gadolinia fuel rods are disposed in the positions other than the outermost periphery. However, the fuel rods positioned at the four corners of the fuel rods group in the second position inwardly from the outermost periphery tend to be the highest in power.
Furthermore, in the fuel assembly constituted of the long-length fuel rods and the short-length fuel rods of which fuel effective portions are shorter than that of the long-length one, a moderator exists in the place of the nuclear fuel material in the shorter portion of the short-length fuel rod. Accordingly, the power of the fuel rods adjacent thereto tends to become high.
Thus, in the gadolinia fuel rods disposed in the positions shown in the aforementioned (1) to (4), since the power tends to become high, the uranium enrichment is necessary to be lowered in comparison with the fuel rods disposed in the other positions.
However, in recent years, from a viewpoint of an improvement in fuel economy or the like, a higher burnup of the reactor core is aimed and the uranium enrichment averaged over the fuel assembly is demanded to be further higher. In the D lattice in particular, the neutron flux distribution in a diameter direction and the enrichment distribution do not coincide to be poor in fuel economy. Accordingly, there is a latent necessity in improving the enrichment. In spite of these, from the restriction on critical safety of a fuel manufacturing plant or the like, there is a restriction on the uranium enrichment applicable to the fuel pellet, that is, the maximum of 4.9 wt %. Accordingly, in the fuel assembly for higher burnup, to further increase the uranium average enrichment, it is necessary to increase the enrichment of uranium contained in the gadolinia fuel rods.
Still further, in recent years, in order to effectively use plutonium produced by conversion of uranium, a fuel assembly loaded with uranium/plutonium mixed-oxide fuel (MOX fuel) in which plutonium obtained by reprocessing spent fuel is enriched is disclosed.
The present invention was carried out to solve these problems. An object of the present invention is to provide a fuel assembly and a nuclear reactor. The fuel assembly is capable of reducing a residual reactivity of a poison at the cycle end and further capable of sufficiently increasing an enrichment of a fissile material. The nuclear reactor is provided with such fuel assembly and improved in the thermal margin. Furthermore, another object of the present invention is to provide a fuel assembly and nuclear reactor in which without damaging thermal margin of the gadolinia fuel rods, a bundle-averaged enrichment of a fissile material is increased to result in an improvement of fuel economy.
A first aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as a burnable poison, wherein at least one of the fuel rods having gadolinium contains gadolinium enriched in at least one kind of isotope of odd mass number more than an isotopic abundance of natural gadolinium, and in the enriched gadolinium, a ratio of a content of gadolinium of mass number of 155 to that of gadolinium of mass number of 157 is 0.1 or less.
In the present fuel assembly, the fuel rods comprising gadolinium in which the isotope of mass number 157 alone is enriched more than an isotopic abundance in the natural gadolinium may be used. Alternatively, the fuel rods comprising gadolinium in which gadolinium isotopes of mass numbers 157 and 155 are enriched respectively may be used. By making the ratio of a content (wt %) of gadolinium of mass number 155 to that of gadolinium of mass number 157 0.1 or less, performance of the fuel assembly can be improved.
A second aspect of the present invention is a fuel assembly comprising a plurality of fuel rods that are bundled in grid pattern and contain enriched uranium as a nuclear fuel material. A part of the fuel rods contains gadolinium as a burnable poison. At least one of the fuel rods having gadolinium contains the gadolinium in which at least one kind of isotopes of odd mass number is enriched more than the isotopic abundance of natural gadolinium. A concentration (wt %) G0 of an oxide of the enriched gadolinium averaged over the entire fuel rods having the gadolinium is set in the range shown by the following expression.
G less than 0.25xc2x7Pxc2x7M/W
In the above expression, M denotes the number of month per one cycle under rated power operation of an equilibrium reactor core, P denoting a power density (kw/l unit) of the nuclear reactor, W denoting a sum of the isotopic composition (% unit) of the enriched gadoliniums of odd mass number, respectively.
The aforementioned expression (G less than 0.25xc2x7Pxc2x7M/W) restricts an average concentration of enriched gadolinia that is an oxide of the enriched gadolinium. The above expression is obtained by varying respectively the isotopic composition (weight ratio) of gadolinium isotopes (Gd-155 and Gd-157) of odd mass number and the gadolinia concentration to evaluate. The upper limit of an appropriate average gadolinia concentration is proportional to the number of operation month M and the power density P, respectively, being inversely proportional to the sum of the compositions of the isotopes of Gd-155 and Gd-157. A factor 0.25 that is a proportional constant is obtained by repeating evaluations.
Thus, in the fuel assembly in which gadolinium isotopes of odd mass number are enriched, when the gadolinia average concentration G0 satisfies the above expression, the residual reactivity of gadolinia at the operation cycle end can be made the minimum and the excess reactivity can be appropriately suppressed.
In the above expression (G less than 0.25xc2x7Pxc2x7M/W) setting an appropriate range of the gadolinia concentration, uranium is assumed as a fissile material. Even in the case where a fuel of mixed oxide (MOX fuel) of uranium and plutonium is used, the gadolinia enriched in the gadolinium isotopes of odd mass number can be applied. In the fuel assembly where the MOX fuel is used, the gadolinia average concentration can be set at one half the gadolinia average concentration in the fuel assembly where uranium is used.
That is, the fuel assembly comprises a plurality of fuel rods that are bundled in grid pattern and contain mixed oxide of uranium and plutonium as a fissile material. A part of the fuel rods contains gadolinium as the burnable poison. Here, at least one of the fuel rods having gadolinium contains the gadolinium in which at least one kind of isotopes of odd mass number is enriched more than the isotopic abundance in natural gadolinium. Furthermore, the concentration (wt %) G0 of an oxide of the enriched gadolinium, averaged over the entire fuel rods having the aforementioned gadolinium is set in the range shown by the following expression.
G less than 0.15xc2x7Pxc2x7M/W
In the expression, a factor 0.15 is obtained by repeating evaluations with the fuel assembly that uses the MOX fuel.
Further, in an initial loading core where uranium is used as the fissile materials the reactivity of gadolinia needs to be maintained for two cycles. In that case, the upper limit of the gadolinia average concentration can be set at two times that at the equilibrium core having uranium.
That is, a third aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern and containing enriched uranium as the nuclear fuel material. A part of the fuel rods contains gadolinium as the burnable poison. Here, at least one piece of the fuel rods containing gadolinium contains the gadolinium in which at least one kind of isotopes of odd mass number is enriched more than the isotopic abundance of natural gadolinium. Furthermore, the concentration (wt %) G0 of an oxide of the enriched gadolinium averaged over the entire fuel rods having the aforementioned gadolinium is set in the range shown by the following expression.
G less than 0.5xc2x7Pxc2x7N/W
In the above expression, N denotes the number of month per one cycle under the rated power operation of the initial loading reactor core, P denoting the power density (kw/l unit) of the nuclear reactor, W denoting a sum of the isotopic composition (% unit) of the enriched gadoliniums of odd mass number, respectively.
In the fuel assembly of the present invention, the fuel rod containing the oxide (enriched gadolinia) of the enriched gadolinium can have a plurality of segments different in the enriched gadolinia concentration. The gadolinia fuel rod having such plurality of segments of different enriched gadolinia concentrations may have a boundary portion in which a difference of the gadolinia concentrations between adjacent segments is in the range of 0.5 wt % or more, preferable to 0.5 wt %xcx9c1.0 wt %.
Furthermore, the fuel rod containing the enriched gadolinia may comprise a plurality of segments of different gadolinia concentrations and an intermediate segment having a gadolinia concentration lower by 1.0 wt % or more in comparison with the adjacent two segments. Here, a length of the intermediate segment can be set at one twenty-fourth or less the effective length of the gadolinia fuel rod.
In addition, in the gadolinia fuel rod having a plurality of segments of different enriched gadolinia concentrations, the gadolinia concentration of the lowermost segment may be made the largest.
In the present fuel assembly, the fuel rod that contains no gadolinia and the gadolinia fuel rod containing enriched gadolinia have segments containing natural uranium at the upper and lower ends thereof, respectively. In addition, the length of the natural uranium segment at the upper end of the gadolinia fuel rod can be longer than that of the natural uranium segment at the upper end of the fuel rod that contains no gadolinia.
Further, the gadolinia fuel rod having the enriched gadolinium can be configured so as to comprise the natural uranium segments at the upper and lower ends each, and so that the length of the natural uranium segment at the upper end is three twenty-fifth or more the effective length of the gadolinia fuel rod.
A fourth aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as the burnable poison. The fuel assembly is arranged with different spacing from each other thereof in the reactor core. At least one of the fuel rods having the gadolinium contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance in the natural gadolinium. In addition, the fuel rods containing the enriched gadolinium are disposed in the region on the control rod side when divided by a diagonal line.
In the present fuel assembly, the fuel rods comprising gadolinium (gadolinia fuel rod) in which the isotope of mass number 157 (Gd-157) alone is enriched more than the isotopic abundance of the natural gadolinium may be used. Alternatively, the fuel rods comprising gadolinium in which Gd-157 and Gd-155 are enriched respectively more than the isotopic abundance of the natural gadolinium may be used.
In this fuel assembly, the content (isotopic composition of at least one of Gd-157 and Gd-155 of which neutron absorption cross sections are large is higher than the isotopic abundance of the natural gadolinium.
Accordingly, even when designing for the fuel assembly to have the same reactivity controllability, the content of gadolinia can be reduced less than that in one containing the natural gadolinium. As a result, when the gadolinia fuel rods are disposed in the region on the control rod side where the power tends to be high, the enrichment of the fissile material such as uranium or the like need not be reduced excessively. Accordingly, without damaging thermal margin of the gadolinia fuel rods, the enrichment of the fissile material averaged over the fuel assembly can be increased.
A fifth aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern. A part of the fuel rods contains gadolinium as the burnable poison. At least one of the fuel rods having the gadolinium contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium. The fuel rods containing the enriched gadolinium are disposed on at least one of four corners of the fuel bundle in the second position inwardly from the outermost periphery.
In the fuel assembly configured thus, the gadolinia content can be reduced less than that in one having the natural gadolinium as the burnable poison. Accordingly, even when the gadolinia fuel rods are disposed at the positions of four corners of the fuel bundle positioned in the second position inwardly from the outermost periphery where the power tends to be high, it is not necessary that the enrichment of the fissile material such as uranium is reduced excessively. Without damaging thermal margin of the gadolinia fuel rods, the average enrichment of the fissile material can be increased.
A sixth aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern. A part of the fuel rods contains gadolinium as the burnable poison. The fuel rods consist of long-length ones and short-length ones of which fuel effective portions are shorter than those of the long-length ones. In addition, at least at part of the positions adjacent to the short-length fuel rods, the fuel rods containing gadolinium in which at least one kind of the isotopes of the odd mass number is enriched more than the isotopic abundance of the natural gadolinium are disposed.
In the fuel assembly thus configured, the gadolinia content can be reduced less than that in one having the natural gadolinium. Accordingly, even when the gadolinia fuel rods are disposed at the positions adjacent to the short-length fuel rods that tend to be high in power, it is not necessary to reduce excessively the enrichment of the fissile material. Accordingly, without damaging thermal margin of the gadolinia fuel rods, the average enrichment of the fissile material can be increased.
A seventh aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern. A part of the fuel rods contains gadolinium as the burnable poison. At least one of the fuel rods having the gadolinium contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium. The fuel rods containing the enriched gadolinium are disposed in at least one of the four corners.
In the fuel assembly thus configured, the gadolinia content can be reduced less than that in one having the natural gadolinium. Accordingly, even when the gadolinia fuel rods are disposed at the positions of the four corners that tend to be high in power, it is not necessary to reduce excessively the enrichment of the fissile material. Without damaging thermal margin of the gadolinia fuel rods, the average enrichment of the fissile material can be increased.
Now, in the manufacture of uranium pellets, to reduce expense and time as much as possible, it is required to limit the number of kind of the uranium enrichment to the minimum. Without decreasing the uranium enrichment averaged over the fuel assembly, in order to make the kind of the enrichments the minimum, the enrichments in the positions where the power tends to be high in the existing design needs to be increased. For this, it is necessary that the gadolinia fuel rods are disposed in the corresponding positions and the positions adjacent thereto and the output of the fuel rods is suppressed. In this case, gadolinium that is higher in the ratios of the isotopes of odd mass number than in the isotopic abundance of the natural gadolinium can be used. Thereby, without excessively lowering the uranium enrichment of the fuel rod itself, the uranium enrichments of the other fuel rods can be increased to realize uniformity of the enrichment.
That is, an eighth aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern. A part of the fuel rods contains gadolinium as the burnable poison. At least one of the fuel rods having the gadolinium contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium. In addition, all the fuel rods excluding those containing the enriched gadolinium are uniform in the enrichments of the contained fissile material except for the upper and lower ends in the axial direction.
In the fuel assembly thus configured, the gadolinia content can be reduced less than that in one having the natural gadolinium. Accordingly, the gadolinia fuel rods can be disposed at the positions that tend to be high in power. Thereby, the enrichment of the fissile material of the peripheral fuel rods can be increased and the enrichment of the fissile material of the fuel pellet over the entire fuel assembly can be made uniform. In the present situation where the upper limit of the enrichment of the fissile material in the pellet is determined, it is made possible to increase the enrichment averaged over the bundle to the maximum and to improve fuel economy.
Thus, in the fourth through eighth aspects of the present fuel assembly, by employing gadolinium (enriched gadolinium) higher in the isotopic composition of the isotopes of odd mass number than that of the natural abundance, the enrichment of the fissile material is heightened and the fuel economy can be improved.
Now, the aforementioned enriched gadolinium has the following action and effect. That is, the gadolinia concentration is determined so that gadolinium, after the burnout at the operation cycle end, does not cause a reactivity loss. However, the gadolinium isotopes that burn out are Gd-155 and Gd-157 that are large in the neutron absorption cross section and the contents of the other isotopes decrease or increase only a little bit. Accordingly, gadolinium as a whole, the neutron absorption is maintained and this causes the reactivity loss.
However, when for instance Gd-157 is enriched more than its natural abundance, there is only a little neutron absorption due to Gd-158 generated by the neutron absorption of Gd-157, and as a whole the reactivity loss is largely decreased. Accordingly, it is preferable to dispose gadolinium higher in the ratios of the isotopes of odd mass number than in the natural isotopic abundance at the end portions of the axial direction where the power is particularly low to be abundant in gadolinium residues.
A ninth aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern. A part of the fuel rods contains gadolinium as the burnable poison. In at least one piece of the fuel rod contains the gadolinium and the upper and/or lower end portions in the axial direction contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium.
In the fuel assembly thus configured, the reactivity loss at the cycle end can be reduced to improve a burnup efficiency of the fuel.
The gadolinium in which the isotope of odd mass number is enriched more than the natural isotopic abundance has the following action and effect. That is, by increasing the isotopic composition of Gd-157 or the like of which neutron absorption cross section is large, reactivity controllability per one gadolinia fuel rod can be increased to result in a decrease in the number of the gadolinia fuel rods per one fuel assembly.
In general, when the number of the gadolinia fuel rods is decreased, a power peaking coefficient of the fuel rod at the initial stage of burnup becomes smaller and fuel rod power density also becomes smaller, resulting in preferable situation from the core performance. Furthermore, when the gadolinia fuel rods are adjacently disposed in the existing design, due to the decrease of the reactivity controllability, it is necessary that a number of the gadolinia fuel rods is increased. However, in the fuel assembly where the gadolinia fuel rods are adjacently disposed as mentioned above, the gadolinium that is enriched more than the natural isotopic abundance in the isotopes of odd mass number can be used.
A tenth aspect of the present invention is a fuel assembly comprising a plurality of fuel rods bundled in grid pattern. A part of the fuel rods contains gadolinium as the burnable poison. Two or more. pieces of the fuel rods containing the gadolinium are disposed to adjoin through a face. At least one of the fuel rod disposed adjoining through the face contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium.
In the fuel assembly thus configured, even when the gadolinia fuel rods are disposed adjacent, the number of the gadolinia fuel rods can be suppressed. Accordingly, the power peaking coefficient of the fuel rod can be suppressed to insure thermal margin of the fuel rod.
Furthermore, in the design of the fuel assembly, there is a demand of increasing an amount of the fissile material loading for one fuel assembly. When the spacing between the fuel rods is set constant, to make an area that the fuel rods occupy as large as possible, there has been developed a technology where two or more kinds of fuel rods different in diameter are prepared and these are alternately disposed. When a fuel assembly is configured with a plurality of kinds of fuel rods of different diameter, there are other advantages.
In the fuel assembly thus configured, when gadolinia is contained in a fuel rod of smaller diameter, it is necessary to increase a content of gadolinia since in the fuel rod of smaller diameter gadolinium burns out earlier. However, when the content of gadolinia is increased, thermal conductivity becomes lower, resulting in poor thermal margin. However, when gadolinium that is enriched in the isotopes of odd mass number more than the natural isotopic abundance is used for the fuel rods of smaller core diameter, the content of the gadolinia can be reduced.
That is, a fuel assembly comprising a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as the burnable poison, can be configured as follows. That is, the fuel rods have at least two kinds of diameters and at least a part of the fuel rods of the smallest diameter contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium being contained.
In the fuel assembly thus configured, even in the fuel rods of smaller diameter, the content of gadolinia can be suppressed and thereby thermal margin of the gadolinia fuel rods can be insured.
An infinite multiplication factor of the fuel assembly in which gadolinium consists only of Gd-157, in comparison with the case of the natural gadolinium being used, shows a sharp and large peak value. It invites conditions unfavorable to core performance such as deterioration of reactor shut-down margin and an increase of channel peaking. By making the infinite multiplication factor a relatively mild peak, the aforementioned problems can be overcome.
That is, a fuel assembly comprising a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as the burnable poison, can be configured as follows. That is, the gadolinia fuel rods containing the gadolinium have at least two kinds of diameters, and at least a part of the gadolinia fuel rods contains gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium.
In the fuel assembly thus configured, even when the gadolinia content is the same, due to the difference of the core diameter of the fuel rods, the specific burnup where the fuel burns out is different. Accordingly, the specific burnup where the infinite multiplication factor reaches its peak is different depending on the fuel rod, and the variation of the infinite multiplication factor of the fuel assembly as a whole becomes moderate. A deterioration of the reactor shut-down margin and an increase of channel peaking are not caused.
Furthermore, in the technology where gadolinium enriched in the isotopes of odd mass number more than the natural abundance is used, there are the following problems to be solved. That is, there is a case where a difference of the gadolinia concentration in its axial direction is disposed in one fuel rod. In that case, so as to enable to detect, by means of the non-destructive inspection after manufacture, the gadolinia concentration distribution in the axial direction, it is necessary to give the concentration difference of more than the minimum detection limit. By contrast, the isotopes of gadolinium of odd mass number being large in their neutron absorption cross section, their gadolinium contents are low. Accordingly, it is difficult to give the necessary concentration difference. In the present invention, in view of such situation, the fuel assembly can be configured as mentioned below.
That is, a fuel assembly comprising a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as the burnable poison, can be configured in the following way. That is, gadolinium in which at least one kind of the isotopes of odd mass number is enriched more than the isotopic abundance of the natural gadolinium, and the natural gadolinium are used, respectively. In addition, in the gadolinia fuel rod containing the natural gadolinium, there is disposed a difference in the concentrations of the natural gadolinium contained in the segments in the axial direction excluding the upper and lower ends.
In the fuel assembly thus configured, when the gadolinia concentration is differentiated in the axial direction of the fuel rod, since the natural gadolinium is contained relatively much, the concentration difference in the axial direction can be easily given to be detectable. On the other hand, in the positions where the power is large, the gadolinium enriched in the isotopes of odd mass number more than the natural isotopic abundance can be used and there is no necessity of lowering the enrichment of the fissile material.
The fifth through tenth aspects of the present invention can be applicable to the fuel grid pattern other than the D lattice. Furthermore, the sixth aspect can be applicable only to the fuel assembly having the short-length fuel rods, and the other aspects can be applicable whether there are short-length fuel rods or not.
A nuclear reactor of the present invention has any one of the fuel assemblies set forth in the first to tenth aspects and the fuel assembly comprising the natural gadolinium, respectively. Here, an average concentration of gadolinium in the fuel assembly comprising the natural gadolinium is larger than that of gadolinium in the fuel assemblies according to the first to tenth aspects.